1. Field of the Invention
The first aspect of the present invention relates to a method and apparatus for sequentially and continuously determining the concentrations of carbon, hydrogen, and nitrogen in molten steel.
The second aspect of the present invention relates to a method and apparatus for rapidly determining trace amounts of carbon in molten steel with no deoxidation or slight deoxidation. The method and apparatus in the second aspect are basically the same as those in the first aspect. More particularly, the second aspect of the present invention relates to a method and apparatus .for rapidly and accurately determining trace amounts of carbon which could not be determined directly by the conventional method. The method and apparatus can be effectively used when dissolved carbon is removed from molten steel with no deoxidation or slight deoxidation using a vacuum decarburizing unit such as RH degassing unit.
2. Description of the Prior Art
It is of the utmost importance in a steel mill to control the concentrations of carbon, hydrogen, and nitrogen in molten steel. In fact, it is necessary to control the concentrations of either nitrogen and hydrogen or either nitrogen and carbon, depending on the refinery involved. Of these three elements, carbon has recently become a matter of great concern. Especially, the determination of carbon concentrations in ultralow carbon steel sheet is attracting considerable attention. The reason for this is explained below.
Ultralow carbon steel sheet has been widely in use mainly for automobiles. As compared with low carbon steel, it is superior in ductility and deep-drawability. On the other hand, it suffers a disadvantage of lacking sufficient mechanical strength. Therefore, many attempts have been made to improve its mechanical strength while maintaining its ductility. This object is achieved by, for example, adding any one or two of such elements as Ti, Nb, Mn, and P. Another important factor to be considered is to control trace amounts of carbon. If it is possible to control trace amounts of carbon, it would be possible to reduce the kinds and amount of additives. For this reason, there is a demand in the steel making industry for a technique to control the concentration of trace amounts of carbon to a precision of the order of ppm in molten steel containing 10-100 ppm of carbon.
The process for producing ultralow carbon steel in a steel mill involves the use of a vacuum decarburizing unit (typified by RH degassing unit). According to this process, decarburization is accomplished by the reaction in vacuo of dissolved carbon in molten steel (with no deoxidation or slight deoxidation) and dissolved oxygen to give carbon monoxide. This is the background to be taken into account in establishing the method of determining trace amounts of carbon in molten steel.
There are some methods for rapid determination of carbon concentration in molten steel. They include freezing point measurement and emission spectrophotometry. Unfortunately, they are not suitable for rapid determination of low carbon concentrations.
A technique is being used on trial to estimate the carbon concentration in the RH degassing unit. According to this technique, CO and CO.sub.2 are sampled from the gas sucked under vacuum from the molten steel and the sample gas is analyzed by means of a mass spectrometer. The cumulative amount of CO and CO.sub.2 indicates the amount of carbon removed. A disadvantage of this technique is that sampling from the vacuum system is difficult to carry out and calculations are subject to errors because the total amount of gas evolved is not known exactly. In addition, leakage from the vacuum vessel presents difficulties in accurately estimating the carbon concentration in molten steel. The lower the carbon concentration in molten steel, the greater the difficulty. No procedure has been established yet to rapidly determine the concentration of trace amounts of carbon.
Although there have been proposed several methods for rapidly determining carbon concentrations, none of them are satisfactory.
In addition to a demand for a method of rapidly determining the concentration of trace amounts of carbon, there is also a demand for an apparatus for continuously determining the concentrations of other elements (such as hydrogen and nitrogen) than carbon contained in molten steel. There may be no instance where it is necessary to determine these three elements simultaneously; however, there does exist an instance in a steel mill where it is necessary to continuously determine two elements, namely nitrogen and carbon or nitrogen and hydrogen.
There is a pioneering technique relating to the determination of several elements, namely carbon, hydrogen, and nitrogen. It is disclosed in Japanese Patent Kohyo No. 502776/1989. It is chiefly intended to determine batchwise the hydrogen concentration. Its procedure consists of blowing a carrier gas (an inert gas) into molten steel for bubbling, recovering the gas, and determining hydrogen in the recovered gas. The results of determination indicate the hydrogen concentration in molten steel. The disclosure suggests that the same procedure can also be applied to the batchwise determination of carbon monoxide and nitrogen in the recovered gas.
The apparatus for this procedure is schematically shown in FIG. 16. It is composed of a probe (150), with its lower part placed in molten steel under examination for gas bubbling and gas collection, and a gas circulating circuit (151), which is made up of a carrier gas supplier and a gas analyzer. The probe (150) is made up of a gas blowing tube (100), with its lower part bent into a U-shape, and a gas collecting tube (101), with its open end positioned above the U-shape. Above the opening of the gas blowing tube (100) is a bell-shaped part (102) of porous material to collect the carrier gas efficiently while preventing the molten steel from entering the gas collecting tube (101).
The gas circulating circuit (151) is made up of a filter (103), thermal conductivity detector (104), pump (105), four-way stopcock (106), pressure gauge (107), and flow meter (108), which are arranged along the gas flow.
The apparatus is operated in the following manner to determine the hydrogen concentration. A carrier gas supplied from a gas cylinder (109) is allowed to bubble in molten steel through the gas blowing tube (100). The carrier gas mixed with dissolved hydrogen in molten steel is collected by the gas collecting tube (101). The collected carrier gas is allowed to circulate through the gas circulating circuit (151) so that the hydrogen concentration in the gas is equilibrated with the hydrogen concentration in the molten steel. Finally, the hydrogen concentration is determined by means of the thermal conductivity detector (104).
If the above-mentioned technique is to be used to determine the concentrations of carbon, hydrogen, and nitrogen, it is necessary to replace the single thermal conductivity detector (104) by a plurality of thermal conductivity detectors arranged in series, each (except the first one) preceded by a filter to remove unwanted gas components. The first thermal conductivity detector measures the total pressures of carbon monoxide, hydrogen, nitrogen, and carrier gas. The second thermal conductivity detector preceded by a hydrogen filter measures the total pressures of carbon monoxide, nitrogen, and carrier gas. The third thermal conductivity detector preceded by a carbon monoxide filter measures the total pressures of nitrogen and carrier gas. The final thermal conductivity detector preceded by a nitrogen filter measures the total pressure of carrier gas alone. Thus, it is possible to obtain the respective partial pressures of carbon monoxide, hydrogen, and nitrogen from the difference in total pressure between the adjacent two stages.
The above-mentioned disclosure suggests the continuous determination of carbon monoxide, hydrogen, and nitrogen; however, it mentions nothing about a concrete means to determine carbon. The complete lack of disclosure about the rapid determination of trace amounts of carbon in molten steel frustrates those who need it in a steel mill.
In addition, the above-mentioned apparatus is designed such that the carrier gas, which after recovery from molten steel contains carbon monoxide, hydrogen, and nitrogen is circulated through the gas circulating circuit, during which these gases are filtered and determined sequentially. This method, however, is impracticable in principle.
The above-mentioned apparatus is useful only when molten steel contains dissolved hydrogen and nitrogen (to be determined) in the form of atoms displaying certain partial pressures in equilibrium at a given temperature and pressure. It is totally unapplicable to the determination of carbon. Carbon in itself present in molten steel does not possess its intrinsic partial pressure at the ordinary refining temperature. Therefore, it is impossible to sample carbon in the form of gas mixed with the carrier gas.
The investigation into the composition of the carrier gas recovered from molten steel revealed that it impossible that hydrogen and carbon monoxide exist simultaneously in the carrier gas. It follows that molten steel containing oxygen in low concentrations gives off no carbon monoxide and conversely molten steel containing oxygen in high concentrations gives off no hydrogen. It was found by the investigation that molten steel gives off hydrogen when its oxygen content is low as in the case of killed steel. This suggests that dissolved hydrogen reacts with oxygen to give water (in the form of steam) when molten steel contains oxygen in high concentrations. It was also found by the investigation that molten steel gives off carbon monoxide when its oxygen content is higher than 200 ppm (as in the case of steel with no deoxidation or slight deoxidation). This suggests that dissolved carbon reacts with oxygen to give carbon monoxide. It follows that molten steel containing oxygen in low concentrations gives off no carbon monoxide because of too small quantities of oxygen available for reaction. It turned out that carbon monoxide is formed by the same reaction as that involved in decarburization by a vacuum refinery. It is apparent from the foregoing that the technology disclosed in the abovementioned Japanese patent cannot be applied to the determination of trace amounts of carbon in molten steel with no deoxidation or slight deoxidation which is the object of the present invention.
Moreover, the above-mentioned prior art technology has a shortcoming in continuously determining hydrogen and carbon monoxide using the carrier gas common to them. That is, if hydrogen enters the carrier gas in the presence of dense oxygen, it reacts with oxygen in the carrier gas to give water, making it impossible to determine the concentration of hydrogen, which is the prime object. To avoid this, it is necessary to determine the concentration of hydrogen in the presence of rarefied oxygen. By contrast, it is necessary to determine the concentration of carbon monoxide in the presence of dense oxygen because carbon monoxide does not form in the presence of rarefied oxygen.
As mentioned above, the prerequisite conditions for the determination of hydrogen concentration differ from those for the determination of carbon concentration. Therefore, it is unjustifiable to use the carrier gas recovered from molten steel for determination of both hydrogen concentration and carbon concentration.
In the case where the concentration of a specific element is to be determined by a thermal conductivity detector, it is desirable for accurate determination that there be as great a difference as possible in thermal conductivities between the element and the carrier gas. Unfortunately, there is a large difference in thermal conductivities between hydrogen and nitrogen and hence it is difficult to use a single carrier gas for determination of both hydrogen concentration and nitrogen concentration. Some device is necessary to overcome this difficulty.
Since the technique disclosed in the above-mentioned Japanese Patent Kohyo No. 502776/1989 cannot be used as such for the continuous determination of carbon, hydrogen, and nitrogen, it is common practice to install several analyzers, each designed to determine the concentration of one element only, in order to determine the concentrations of several elements in a steel mill. Operation of several analyzers needs a lot of time and trouble, which prevents the results of determination from being used effectively for feedback control.